RFC3339 instead of ISO8601, perhaps?
Of course, abusers will only pay attention to this if it benefits them and it's
cheap to do so. But they won't be distinguishable from legitimate clients that
just don't know about this extension and retry by their own schedules, so one
can't penalize such clients for not respecting the request. On the other hand,
you might be able to identify "good" clients (for some value thereof) by
observing which ones do respect the request.
From: owner-ietf-smtp(_at_)mail(_dot_)imc(_dot_)org
[mailto:owner-ietf-smtp(_at_)mail(_dot_)imc(_dot_)org] On Behalf Of Keith Moore
Sent: Tuesday, October 11, 2011 11:06 AM
To: Steve Atkins
Cc: SMTP Interest Group
Subject: Re: We need an IETF BCP for GREY LISTING
On Oct 11, 2011, at 1:45 PM, Steve Atkins wrote:
In that case, some minor extension to allow the SMTP server to communicate
something a bit more nuanced than "Go away, come back later." might have some
value.
I could see value in that. I could imagine an SMTP extension which, if
included, indicated that the server might send a response of the form
4xx please retry between <date-time>-<date-time>
...in response to say the MAIL (or maybe DATA) command, where <date-time> could
be an ISO8601 date-time (the horror!) based on GMT (Z) and with no punctuation.
If the client included a SIZE the server could even do bandwidth reservation.
Of course there would be no guarantee that the second attempt wouldn't result
in some sort of 4xx response for other reasons.
Greylisting servers could certainly make use of it, though I don't know if it
would be a good idea to recommend that greylisting servers use it.
Keith